In the present power tools, such as electric screw drivers, which is driven by a rotary motor and screw into a board of wood via supplying current from a loaded power. The screws may have different diameters. And the shapes of the heads of different screws are different from each other. Thus different circumstance will occur when different screws are driven into the same board of wood. Additionally, if the same screw is driven into different material, the hardness of different pieces of wood may be different, current of the electric screw drivers will be different. Normally, during using of the screw drivers, the user should drill the screw head up to a surface close to the workpiece, thereby, the user needs to watch the drilling process very carefully in order to stop the motor when the screw head reaches the end. Thus, the screw may not go too deeply into the piece of wood what it undesirable. Otherwise, it can prevent the motor from overloading when the screw head is incautiously screwed into the wood board.
It is prior art to use some overload protection device. This may be a mechanical clutch, which disengage the tool bit from the driving motor when the motor current exceeds a predetermined limit value. A torque collar, which is rotatable and marked with plurality of scales, is provided in the front of the housing of which power tool has such overload protection device. These scales indicate the torque limiter. The user should preset the limited torque value by the rotatable torque collar, that is, when the output torque reaches or exceeds the predetermined value, the clutch will automatically start to disengage the tool bit of the power tool and the motor. Additionally, electric screwdrivers with such overload protection device has a sleeve extending from the front end of the housing, the front end of the sleeve is align with that of the tool bit of the electric screwdriver. When the screw head reaches the wood board surface, the front end of the sleeve also reaches it, if the screw is pressed further, the sleeve will be biased to actuate the clutch in the housing, thereby disengage the tool bit and the motor. However, the above mechanical clutch is complex in structure and it is complicated and costly to be made.